Sonar systems that make it possible to recognize the shape of the ocean bottom or to detect the presence of an object on the bottom or floating near the bottom are known in the art. These sonars transmit pulses and receive echoes by means of arrays, comprised of banks of electroacoustic transducers. The receiving transducer is coupled to a device to form monitoring channels, each corresponding to a given position. An object located close to the ocean bottom will send back an echo and also a "shadow" by projection on the ocean bottom. The electrical signals corresponding to the echoes picked up on each channel are displayed on the screen of the cathode ray tube and the shadow projected by the object appears by contrast.
Prior sonar systems make it possible to recognize the presence of objects but do not enable one to identify such objects with precision, due to the poor quality of the image obtained. This poor image quality is caused by the weak distance and angular resolution power provided by previous systems.
Distance resolution is dependent on the width of the acoustic pulses produced by the transmitting transducer. To improve the quality of the images, the pulse width should be reduced. However, a reduced pulse width means that less acoustic energy is transmitted and echoes of less energy are received. Thus, a gain in resolution is offset by a loss in sonar range. This reduction of received energy is aggravated by the fact that it is necessary to use very short ultrasonic waves to reduce transducer dimensions. The angular width of the acoustic beams picked up, which determines the resolution in bearings, depends directly upon the ratio of the dimensions of the transducer, or antenna base, to the length of the acoustic wave used, the angular width being narrower as the aforementioned ratio increases.
Another important difficulty arises from the fact that the range of sonars is generally such that the scan of the receiving transducer takes place in a close field within the Fresnel region of the transducer, i.e., the region in which the acoustic waves are not planar. This causes the acoustic beam picked up by the receiving transducer to have a relatively large angular width, and high level side lobes which considerably limits the bearing resolution power and the contrast and makes recognition of small objects uncertain.